The invention relates to a furnace for (thermally) cracking a hydrocarbon feed in the vapour phase in the presence of steam. The invention further relates to a method for (thermally) cracking a hydrocarbon feed in the vapour phase in the presence of a diluent gas, in particular steam.
Cracking furnaces are the heart of an ethylene plant. In these furnaces, feeds containing one or more hydrocarbon types are converted into a cracked product gas by cracking of hydrocarbons. Typical examples of hydrocarbon feeds are ethane, propane, butanes, naphtha's, kerosenes and atmospheric and vacuum gasoils.
Processes for converting hydrocarbons at higher temperature have been known for many decades. U.S. Pat. No. 2,182,586, published in 1939, describes a reactor and process for the pyrolytic conversion of a fluid hydrocarbon oil. Use is made of a horizontally arranged single reactor pipe (the publication refers to “tubes”, but these are connected in a serial flow connection and thus form in fact a single tube), which results in relatively long residence times which are common in the process of thermal cracking of liquid hydrocarbon oils to improve motor fuel quality such as visbreaking. The use of the described heater for a process like steam cracking or for the cracking of a vaporous feed is not mentioned. Rather, excessive cracking and excessive gas formation are avoided.
U.S. Pat. No. 2,324,553, published in 1943, shows another heater for the pyrolytical conversion of hydrocarbons, wherein the reactor pipe is formed of serially connected “tubes”, which are horizontally positioned in the heater. In the described process, oil is passed through the tube to a temperature below an active cracking temperature.
WO 97/28232 describes a cracking furnace for thermally cracking a liquid hydrocarbon feed in a spiral pipe. The furnace is said to have a reduced sensitivity for coke formation and an increased liquid residence time. It is not disclosed to use the installation for steam cracking.
Steam cracking is a specific form of thermal cracking of hydrocarbons in the presence of steam with specific process kinetics and other process characteristics. Herein, the hydrocarbon feed is thermally cracked in the vapour phase in the presence of steam. The cracking is carried out at much higher severity than applied in the moderate cracking of liquid hydrocarbon oils to improve fluid quality. Steam cracking furnaces comprise at least one firebox (also known as a radiant section), which comprises a number of burners for heating the interior. A number of reactor tubes (known as cracking tubes or cracking coils) through which the feed can pass, are disposed through the firebox. The vapour feed in the tubes is heated to such a high temperature that rapid decomposition of molecules occurs, which yields desired light olefins such as ethylene and propylene. The mixture of hydrocarbon feed and steam typically enters the reactor tubes as a vapour at about 600° C. In the tubes, the mixture is usually heated to about 850° C. by the heat released by firing fuel in the burners. The hydrocarbons react in the heated tubes and are converted into a gaseous product, rich in primary olefins such as ethylene and propylene.
In cracking furnaces, the reactor tubes may be arranged vertically in one or more passes. In the art, the term cracking coil is also used. One or more of the cracking coils, which may be identical or not identical, may be present to form the total radiant reactor section of a firebox. Conventionally, ethylene cracking tubes are arranged in the firebox in one lane wherein the lane is heated from both sides by burners.
Such a lane may be in a so-called in-line arrangement whereby all the reactor tubes are arranged in essentially the same vertical plane. Alternatively, the tubes in such a lane may be in a so-called staggered arrangement whereby the tubes are arranged in two essential vertical parallel planes whereby the tubes are arranged in a triangular pitch towards each other. Such a triangular can be with equal sides (i.e. equilateral triangular pitch) or with unequal sides which is called an extended pitch.
Examples of such a extended pitch configuration are isosceles triangular pitch, right angled triangular pitch and any other non-equilateral triangular pitch. An example of such a furnace with an extended pitch is GK6™ (see FIG. 1) featuring a isosceles non-equilateral triangular pitch in a dual lane coil arrangement. In the GK6 furnace, the set of two lanes is heated from both sides by burners 5 located in the bottom and/or sidewall. The inlet sections (extending from inlets 4) and outlet sections (extending from outlets 3) are heated essentially equally by the burners 5.
It has been found that this leads to less-optimal cracking conditions. It is thought that this is due to a not so advantageous heat distribution. The cracking process is an endothermic process and requires the input of heat into the feed. For the performance (selectivity) of the cracking process it is desirable to maximise the heat input to the inlet section of the cracking coil (tube). The inventors therefore sought a way to alter the input of heat into the cracking tubes.
In addition, it has been found that the use of a known furnace for (thermally) cracking a hydrocarbon vapour in the presence of steam, thereby forming ethylene, propylene and/or one or more other alkenes (also called olefins), leads to less favourable conditions for mechanical stability of the cracking coil assembly.
The inventors realised that due to the fact that inlet sections at one side of the staggered lane have different temperature conditions and heat distribution conditions than the outlet sections at the other side of the staggered lane, different thermal stress and thermal creep conditions exists between the inlet sections and the outlet sections. Creep is the irreversible expansion which occurs when heating a metal. Creep is the result of thermal stresses inside the metal due to heating. Thermal stress (caused by thermal expansion) is the reversible phenomenon when heating any material. Both phenomena have to be taken care of in the design of the coil and cause the above mentioned restrictions in the cracking coil mechanical layout.
Therefore such a staggered coil arrangement is usually considered less suitable in steam cracking furnaces to convert light hydrocarbon gases such as ethane. In the steam cracking of ethane, due to stiff nature of carbon deposit at the inside of the coil, too much unbalance in thermal stresses and thermal creep may cause tube bending or even coil rupture. However, even with an in-line arrangement conventionally applied in the art of ethane cracking, such an arrangement requires a complicated coil support system at the inlet, outlet and bottom part necessary to compensate for the thermal stresses and thermal creep. This is also the case in cracking heavier vapour hydrocarbons where a sufficient extended staggered arrangement with a properly designed coil support system with variable adjustment parameters could be adequate. However continuous operator attention is required to adjust support system settings in case of different operating conditions and during the operating life of the furnace as coil dimensions and strength change as a consequence of creep over time.
It has been found that the input of heat, in a method for (steam) cracking a hydrocarbon can be altered by designing inlet- and outlet sections of the cracking coils in a specific way.
Further, it has been found that the thermal stability of the coils can be improved by designing the cracking furnace, in particular the inlet- and outlet sections of the cracking coils in the fire box of the furnace in a specific way.